This invention relates to stents used to support arterial and venous conduits in the human body. More particularly, it refers to a tubular stent having a non-uniform structure along its longitudinal length to provide good flexibility and radial strength.
There are four major classes of stents employed in the prior art. These four major classes of stents are described as follows:    1. Coil Stents are made from a single wire. The wire is bent in various ways and formed into a stent. Examples of this type of stent are those shown in U.S. Pat. Nos. 4,969,458; 4,681,110 and 5,824,056.    2. Slotted Tube Stents are laser cut using a tube of either stainless steel, nickel/titanium alloy (NITINOL®), titanium or any other suitable materials. These designs are preprogrammed into a machine language and a laser is used to cut the programs. These stents have a uniform design and a uniform thickness from the beginning to the end of the stent. In other words, the same segment is repeated from one end of the stent to the other. Examples of this type of stent are described in U.S. Pat. Nos. 4,733,665; 4,739,762; 4,776,337 and 4,793,348.    3. Self Expanding Stents are usually braided or knitted with multiple wire filaments such that they have a lower profile when stretched and they expand from a lower profile to a higher profile when unconstrained in the body. These stents are called self-expanding stents and are described in U.S. Pat. No. 4,655,771.    4. Hybrid Stents are similar to slotted tube stents except that they do not have a closed cell structure but have an open cellular structure with flexible interconnections between each segment of the design. These interconnections provide the flexibility while the segments provide the radial strength and other important properties of the stent. Examples of this stent are described in U.S. Pat. Nos. 5,514,154; 5,562,728; 5,649,952 and 5,725,572.
In use, each of the four classes of stents described above are coated as described in various patents as follows:    1. U.S. Pat. No. 5,759,174, describes a balloon that has a radiopaque segment attached to one of the longitudinal ends of the balloon. When the balloon is inflated, the stent is pressed against the ends of the artery and this indicates the center portion of the dilated stenosis. The external radiopaque marker band is typically made from a dense radiopaque metal such as tantalum, gold, platinum or an alloy of those dense metals.    2. U.S. Pat. No. 5,725,572, describes gold plating on the ends of a stent such that the gold plating marks two bands at the ends of a stent. The patentee mentions that the limitation of gold coating is the stiffening of the stent surface. Hence, the gold plating is done only at the ends where the stiffening does not significantly alter the properties of the stent. Also described is another embodiment where only the exterior of the stent is coated with a radiopaque material.    3. U.S. Pat. No. 5,919,126, describes a patent where the body of the stent is formed from a non-radioactive structural material, a radiopaque material coats the body and a beta emitting radioisotope ion is implanted into the radiopaque material.    4. U.S. Pat. No. 5,824,056 describes an implantable medical device formed from a drawn refractory metal having an improved biocompatible surface. The method by which the device is made includes coating a refractory metal article with platinum by a physical vapor deposition process and subjecting the coated article to drawing in a diamond die. The drawn article can be incorporated into an implanted medical device without removing the deposited material.    5. U.S. Pat. No. 5,824,077 describes a stent which is formed of multiple filaments arranged in two sets of oppositely directed helical windings interwoven with each other in a braided configuration. Each of the filaments is a composite including a central core and a case surrounding the core. The core is formed of a radiopaque material while the outer casing is made of a relatively resilient material, e.g., a cobalt, chromium based alloy. Alternative composite filaments described in the patent employ an intermediate barrier layer between the case and the core, a biocompatible cover layer surrounding the case, and a radiopaque case surrounding the central core.    6. U.S. Pat. No. 5,871,437 describes a non-radioactive metallic stent which is coated with a biodegradable or non-biodegradable thin coating of less than about 100 microns in thickness which is selected to avoid provoking any foreign body reaction. This coating contains a radioactive source emitting Beta particles with an activity level of approximately one micro curie and on top of this layer is an anticoagulant substance to inhibit early thrombus formation.    7. U.S. Pat. No. 6,099,561 describes a stent having a biocompatible metal hollow tube constituting a base layer having a multiplicity of openings through an open ended tubular wall thereof, the tube constituting a single member from which the entire stent is fabricated. A thin tightly adherent intermediate layer of noble metal overlies the entire exposed surface area of the tube including edges of the openings as well as exterior and interior surfaces and ends of the wall. A third outermost ceramic like layer composed of an oxide, hydroxide or nitrate of a noble metal is formed atop and in adherent relation to an intermediate layer.    8. U.S. Pat. No. 5,722,984 describes a stent which has an antithrombogenic property and contains an embedded radioisotope that makes the coating material radioactive.    9. Other relevant patents that describe the coating technology or coating properties include U.S. Pat. Nos. 5,818,893; 5,980,974; 5,700,286; 5,858,468; 5,650,202 and U.S. Pat. No. 5,696,714.
Although some of the above mentioned stents have good flexibility and others have good radial strength, there is no optimum stent in the prior art that has both good flexibility and radial strength together with the ability to retain a useful coating.